09 December 2007

Human Origins: Wrong Answers to the Question

Anyone who's spent enough time discussing human origins is bound to have heard this old chestnut at some point:

If we're descended from apes, how come apes are still around?

A variant uses "monkeys" instead of apes.

In an attempt to defend the scientific explanation, people will often use one of two stock answers:

If you're descended from your parents, how come they're still around?

We're not descended from apes, but we share a common ancestor with them.

Number 1 has a valid point, but it's a bad analogy. Stem-humans originated around 5–7 million years ago, but there aren't any apes sitting around today that are that old. Our parents are still alive (or were not long ago), but our ape ancestors are long dead.

Number 2 is flat-out wrong. We are descended from apes. You could modify that sentence a bit, though, and get a true statement: "We're not descended from any living apes, but we share a common ancestors with them."

So what's the real answer?

The real answer is that "ape", as commonly used, is an unnatural term, and it obscures communication about the subject. Generally "ape" is used to refer to all hominoids except for humans. What's a hominoid? Let's spell that out in some detail:

(Yes the scientific name of western gorillas really is Gorilla gorilla. And the western subspecies thereof is Gorilla gorilla gorilla!)

As you can see, hominoids form a big, bushy branch of the primate tree, and most of the extant twigs (i.e., species) are among the gibbons (although most living individual hominoids are human—nearly all of the other species shown here are threatened or endangered).

Among living hominoids, we humans are most closely related to chimpanzees (including bonobos). The second runner-up is gorillas, then orangutans, and then finally that big bushy mass of gibbons. These relationships reflect events in our ancestry: the human-chimp split (about 5–7 million years ago), the split from gorillas (about 8–10 million years ago), the split of African hominids and orangutans (about 12–15 million years ago), and the split of great apes and gibbons (about 16–21 million years ago). Each split stems from a population, a group of common ancestors which we share with some other living hominoid group. Those ancestors were hominoids and were not humans; therefore, they were apes.

But, looking at this tree, you can see how unnatural the designation "ape" is. What sense does it make to lump our closest living relatives (chimpanzees) in a group with much more distant relatives (e.g., gibbons)? It would make as much sense as if we invented a group that included all hominoids except for siamang giboons, or all hominoids except for gorillas, or all hominoids except for any randomly-selected group in that cladogram.

(This also points out some problems with the Planet of the Apes movies—chimpanzees would probably just see humans, gorillas, and orangutans as belonging to some group of non-chimp hominoids. And gorillas and orangutans would respectively exclude themselves from such a group, but include chimps. Come to think of it, only the orangutans would be thinking of a natural group.)

Really, the only way to make the term "ape" natural would be to include ourselves. Then we would not only be descended from apes—we would be apes.

The term "monkey" has a similar problem, since some monkeys are more closely related to us than they are to some other monkeys.

Cladogram of dry-nosed primates (haplorrhines), to an arbitrary level of resolution.

A "monkey" is basically any simian that is not an ape (or human). To make it a natural term, we would have to include apes (including humans).

Should we use those terms differently, and consider ourselves apes and monkeys? It would be more honest, phylogenetically speaking. In the end, though, those are vernacular terms and people are free to use them however they want.

Thanks! Of course, if you wanted to be technical, you could use "simian" or "anthropoid" instead of "monkey", and "hominoid" instead of "ape".

I should also point out that "great ape" is an unnatural designation that could be "naturalized" by adding ourselves. Then it would be synonymous with "hominid".

Interestingly, Linnaeus himself actually wanted to place humans with apes and monkeys in Simia, but created a separate genus for us (Homo) because he feared the more anatomically honest scheme would not be popular.

Also interesting is that Linnaeus placed chimpanzees in Homo, not Simia. Of course, he had never actually seen one and was relying on relayed accounts. Who knows what he would have thought if he had an actual specimen? (Then again, who cares -- he was a botanist, not a primatologist.)

Yes! - I find it really irritating when people say we only share a common ancestor with apes in order to argue with creationists. Firstly because, as you point out, it's wrong; and secondly, even if it were true - it's miserly and pedantic to make such a point.

We did evolve from something just about everybody would call and ape, and we further back, we evolved from something everyone would call a monkey.

The question isn't perfect, but I don't like evolutionists acting like it's the dumbest question ever. I think there is some merit to it that could inspire worthwhile discussions. One gets the impression from the fossil record that in pre-human times, there was a great diversity of apes. Today, there aren't as many non-human apes, and they're usually endangered due to human activity. I can see why people might hypothesize a negative correlation between human success and non-human ape success. Were Sahelanthropus, Orrorin, Ardipithecus, Australopithecus, Paranthropus, et al. driven to extinction by competition with their more human relatives/descendants? And if our lineage caused the extinction of all those apes, well, what's different about the apes that have managed to survive despite our presence? Is Pan actually better ecologically suited to coexistence with Homo than Australopithecus was? Of course, this is in the paradigm of a cladogenesis model of hominin evolution... If you want to argue anagenesis, I suppose some of the above makes less sense. :P

When cladogenesis happens, how typically does the descendant species contribute to the extinction of the parent species? Are most extant species the descendants of extinct species, or other extant species? This actually looks like a pretty big and important topic for evolutionary biologists to study.

Well, it's not a very good question if you have a clear understanding of what "ape" means, but, certainly, if you didn't, it might make some sense.

I think though, it assumes that we have changed and apes haven't. Clearly people asking this question have not stopped to notice that, hey, apes all look really different. Ergo, somebody had to have changed.

I'm not sure if ape diversity is down all that much now. Remember, a lot of these different fossil ape species lived at different times. Of course, if current trends continue, apes will be getting a lot less diverse pretty quickly. And certain ape groups, such as Hominina (the human total group) are unquestionably very low in terms of diversity right now. But we don't have a very good record of most ape lineages -- mainly just the one that left the rain forest (where things hardly ever fossilize) for the savannah.

Good point about Pan vs. Australopithecus when it comes to competing with Homo. I think Pan hasn't had to deal with much competition from Homo until relatively recently. Australopithecus, on the other hand, seems to have had much more ecological overlap with Homo. (Maybe that's a naïve interpretation, but it's all I got right now.)

Your question about species really depends on which species concept you're using. Under the phylogenetic concept, a parent species becomes extinct as soon as any daughter species appear, by definition.

I actually tend to take an organism-level approach to cladogenesis, which pretty much ignores the concept of species. (See my paper in Zoologica Scripta for details.)